Gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel and igniting the mixture, and a turbine blade and vane assembly for producing power. Combustors operate at high temperatures that may exceed 2,500 degrees Fahrenheit. Typical turbine combustor configurations expose the turbine vane and blade assemblies to these high temperatures. Turbine vanes and blades must be made of materials capable of withstanding such temperatures. Turbine vanes and blades often contain cooling systems for prolonging their life and reducing the likelihood of failure as a result of excessive temperatures.
A turbine blade is a rotating airfoil attached to a disk on the turbine rotor by a platform and blade shank. A turbine vane is a stationary airfoil that is radially oriented with respect to a rotation axis of the turbine rotor. The vanes direct the combustion gas flow optimally against the blades. One or each end of a vane airfoil is coupled to a platform, also known as an endwall. A radially outer vane platform is connected to a retention ring on the engine casing. An inner vane platform, if present, is supported by the vane.
Blades and vanes often contain cooling circuits forming a cooling system. The cooling circuits receive a cooling fluid such as air bled from the compressor of the turbine engine via a plenum and supply port in one or each platform. The cooling circuits often include multiple flow paths inside the airfoil designed to maintain all portions of the airfoil at a relatively uniform temperature. At least some of the air passing through these cooling circuits may be exhausted through film cooling holes in the leading edge, trailing edge, suction side, and pressure side of the airfoil.
Some turbine airfoils have a dual wall structure formed of inner and outer walls. This is called a 4-wall airfoil construction, since the pressure and suction sides of the airfoil each have two walls. The outer wall is exposed to hotter temperatures, so it is subject to greater thermal expansion, and stress develops at the connection between the inner and outer walls.
It is known that high cooling efficiency can be achieved by near-wall cooling in which cooling air flows in channels between the inner and outer walls of a 4-wall airfoil. However, differential thermal expansion between the hot outer walls and the cooler inner walls can cause Low Cycle Fatigue (LCF) limitations for reasons later described.